Some industries require work to be performed in hazardous locations that have atmospheres containing dangerous concentrations of flammable gases or dust. For example, such hazardous locations can be found in industrial locations where industrial paint and fluid handling systems are used. Such flammable gases and dust can be ignited by providing heat or electrical sparks sufficient to ignite the explosive atmospheres.
The International Electrotechnical Commission (IEC) is an international organization that has promulgated various standards of safety for conducting operations in such hazardous locations. Three such standards are IEC 60079-0, IEC 60079-11 and IEC 60079-25, which are hereby incorporated by reference. Hazardous locations are defined by the IEC as “hazardous areas”. A hazardous area is an “area in which an explosive atmosphere is present, or may be expected to be present, in quantities such as to require special precautions for the construction, installation and use of electrical apparatus.” (See e.g., IEC 60079-0, definition 3.2). An explosive atmosphere is a “mixture with air, under atmospheric conditions, of flammable substances in the form of gas, vapour, dust, fibres, or flyings which, after ignition, permits self-sustaining propagation.” (See e.g., IEC 60079-0, definition 3.30).
Electrical equipment that is operated at such hazardous locations can present such risks of ignition of the flammable gases or dust. Various methods can be used to guard against ignition of such explosive atmospheres. These include using explosive-proof chambers, immersion of electrical equipment is oil or sand, safe design of electrical equipment, and others. Safe design of such electrical equipment to guard against these risks is termed Intrinsic Safety (IS). Electrical equipment designed with Intrinsic Safety (IS) considerations limit the energy, electrical and thermal, available for ignition of the explosive atmospheres. Intrinsic safety is defined as a “type of protection based on the restriction of electrical energy within equipment and of interconnecting wiring exposed to the explosive atmosphere to a level below that which can cause ignition by either sparking or heating effects.” (See e.g., IEC 60079-11, definition 3.1.1).
In normal operation, some electrical equipment can create electric arcs, for example, in switches, motor brushes, connectors, and in other places. Electrical equipment also can generate heat, which in some circumstances can become an ignition source. Even if equipment, in normal operation, doesn't generate ignition sources, various component failures can render such equipment as having the potential to produce such ignition sources. For example, if a component fails in a short-circuit or an open-circuit manner, circuitry that was previously incapable of producing an ignition source can become capable of producing such a source.
A device termed intrinsically safe is designed to be incapable of producing heat or spark sufficient to ignite an explosive atmosphere, even if the device has experienced one or more component failures. Intrinsically safe electrical devices are designed to operate with low voltage and current, and are designed without any large capacitors or inductors that could discharge a spark. Even if a device is intrinsically safe, however, such devices are capable of producing sources of ignition if the power provided to the intrinsically safe electrical device is excessive.
Thus, power-supply barriers are designed that operate in normal locations and provide safe electrical power to devices in hazardous locations. Normal locations are defined by the IEC as “non-hazardous areas.” A non-hazardous area is an “area in which an explosive atmosphere is not expected to be present in quantities such as to require special precautions for the construction, installation and use of electrical apparatus.” (See e.g., IEC 60079-0, definition 3.3). Safe electrical power is power is achieved by ensuring that only low voltages and currents that are present in such hazardous locations, and that no significant energy storage is possible. One of the most common methods for providing safe electrical power is to limit electric current by using series resistors, and limiting the voltage with Zener diodes.